Development means and methods for developing electrostatic images

ABSTRACT

THIS A METHOD FOR DEVELOPING ELECTROSTATIC IMAGES ON A MOVING INSULATING SURFACE WITH DEVELOPER MATERIAL IN WHICH THE FLOW OF DEVELOPER MATERIAL MOVES DOWNWARD AND IN A DIRECTION OPPOSITE TO THE MOVEMENT OF THE INSULATING SURFACE AND WHICH CAN BE TERMED COUNTERFLOW CASCADE DEVELOPMENT. THE FLOW OF DEVELOPER MATERIAL IS DIRECTED ONTO THE INSULATING SURFACE SUCH THAT A FINISHING DEVELOPMENT ZONE IS FORMED AT THE POINT OF ENTRY OF THE DEVELOPER MATERIAL TO A POINT UPWARD THEREFROM. IN ADDITION, MEANS HAVING A CONTROLLING FORCE OVER TONER DEPLETED CARRIER PARTICLES IS POSITIONED ADJACENT THE FINISHING DEVELOPMENT ZONE FOR REGULATING SUCH PARTICLES AND THEREBY PREVENTING THEM FROM BEING CARRIED UPWARDLY AND OUT OF THE ZONE.

May 30, 1972 E. w. LUTTMAN ETA!- 3,666,518

l)1:.Vl;L0l'MHN1' MEANS AND METHODS FOR DEVELOPING ELECTROSTATIC IMAGES Filed March 31, 1970 PRTOR ART JIIVEIUWJES.

ERWIN W. LUTTMAN HAROLD C. MEDLEY 23M WQ'ZGM ATTORNEY United States Patent Oflice 3,666,518 Patented May 30, 1972 US. Cl. 11717.5 7 Claims ABSTRACT OF THE DISCLOSURE This is a method for developing electrostatic images on a moving insulating surface with developer material in which the flow of developer material moves downward and in a direction opposite to the movement of the insulating surface and which can be termed counterflow cascade development. The flow of developer material is directed onto the insulating surface such that a finishing development zone is formed at the point of entry of the developer material to a point upward therefrom. In addition, means having a controlling force over toner depleted carrier particles is positioned adjacent the finishing development zone for regulating such particles and thereby preventing them from being carried upwardly and out of the zone.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to electrostatography and, more particularly, relates to novel development means and methods for developing electrostatic images.

Description of the prior art In an electro'statographic process known as electrophotography, an electrostatic latent image is formed on a photoconductive insulating member and is developed or made visible by the attraction thereto of finely divided pigmented material, commonly known as toner. One prior art development method for rendering the electrostatic image visible is known as parallel flow cascade development in which a developer material is poured or cascaded over the surface of the photoconductive insulating member, while it is moving in a parallel direction with the developer material, as shown in the figure marked prior art. This developer material contains a mixture of particles known in the art as carrier and much smaller pigmented powder particles or toner which are charged by and electrostatically adhere to the carrier particles. As the developer mixture cascades over the electrostatic latent image on the surface of the photoconductive insulating member, the toner patricles are attracted to the electrostatic image from the carrier particle's and deposited on the photoconductive insulating member to render the electrostatic latent image visible.

In parallel flow development as shown in the prior art figure, the velocity of the cascading development material must exceed the velocity of the moving photoconductive member for high quality image development. When the velocity of the photoconductive member is only four inches per second, the velocity of the developer material can easily exceed the velocity of the moving photoconductive member by merely dropping or flowing the developer material onto the member, as shown in the prior art figure. In this situation, an individual electrostatic charge on the member will be contacted by a number of carrier particles full of toner powder for transfer thereto. However, as the velocity of the photoconductive member is increased to higher rates of speed, such as 32 inches per second, the velocity of the photoconductive member now will exceed the velocity of the developer material if it is merely dropped on the member as shown in the prior art figure so that the same individual electrostatic charge will be contacted by carrier particles which are substantially toner depleted because the carrier particles will have previously contacted other electrostatic charges and will have given up substantially all of their toner. In this latter situation, image density of the resultant toner image falls off and other forms of image degradation appear.

Moreover, if the photoconductive member is in the form of a drum or cylinder, as shown in the prior art figure, and the diameter of the drum is decreased, the above effects show up at lower velocities because the electrostatic image is in contact with the developer material for a shorter period of time. Furthermore, there is an upper limit in velocity that the developer material can exceed the velocity of the photoconductive memebr because, in the known parallel flow development method, the development of the electrostatic image begins at the point of entry of the developer material onto the photoconductive member and completed at a point distant from the entry of the development material. Thus, the carrier particles are in a toner depleted state as they leave the developed image, and it their velocity is sufiiciently high, they will scrub oil or remove portions of the developed image, thereby degrading the image quality.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide novel development means and methods which are capable of high speed development of electrostatic images, such as as great as and even greater than 32 inches per second, and which produce high quality toner images.

Another object of the present invention is to provide novel development means and methods which are capable of such high speed development without degradation of the finished toner image.

A further object of the present invention is to provide novel development means and methods in which the background ot the finished toner image is cleaned as the finished toner image goes out of contact with the developer material.

Another object of the present invention is to provide development means and methods which are capable of high speed development, but in which the development material initially contacts the insulating member carrying the electrostatic image at a low velocity, relative to parallel flow development, so as to minimize damage to the insulating member during initial contact therewith.

Another object of the present invention is to provide high speed development means and methods in which the surface of the insulating member is subjected to a polishing action during the development of the electrostatic image, thereby minimizing toner filming of the insulating member.

Another object of the present invention is to provide development means and methods which permits the design of an electrostatographic machine configuration with simplified and convenient paper handling.

The foregoing objects are achieved by a development method in which the flow of developer material moves downward and in a direction opposite to the direction of the insulating surface carrying the electrostatic image and which can be termed counterflow cascade development. The flow of developer material is directed onto the insulating surface such that a finishing development zone is formed at the point of entry of the developer material to a point upward therefrom whereby the development of the electrostatic image is completed in the finishing development zone, but begun at a point a distance from and downward from the point of entry of the developer mate rial. Another aspect of the present invention for achieving the foregoing objects is the provision of means in the flow of development material onto the insulating surface for either deflecting upwardly or restricting downwardly the developer material to cause the formation of the finishing development Zone.

Because the insulating surface carrying the electrostatic image moves in an upward direction, it is possible for toner depleted carrier particles in the upper portion of the finishing development zone to stick to the surface and to be carried upwardly and out of this zone. Such carrier would be carried into other parts of the electrostatographic machine and could cause damage to these parts.

Accordingly, it is a further object of the present inven tion to prevent toner depleted carrier particles from sticking to the insulating surface and being carried upwardly and out of this zone. This object is achieved by providing adjacent the finishing development zone, means having a controlling force over the toner depleted carrier particles for regulating their activity in this zone and thereby prevent the particles from being carried upwardly and out of the zone.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings of which:

FIG. 1 is a schematic cross-sectional view of one embodiment of the development means of the present inventron.

FIG. 2 is a cross-sectional schematic view of another embodiment of the development means of the present invention.

FIG. 3 is a cross-sectional schematic view of the development means with electrical means to show one embodiment of another aspect of the present invention.

FIG. 4 is a cross-sectional schematic view of the development means with magnetic means to show another embodiment of the other. aspect of the present invention.

DESCRIPTION OF THE PRFERRED EMBODIMENTS Referring to FIG. 1, there is partially shown a moving member 11 having an insulating surface 12 carrying an electrostatic charge pattern and moving around a roller 13 in the direction of the arrow 14. Preferably, the member 11 comprises a photoconductive layer and the electrostatic charge pattern is formed by uniformly electrostatically charging the surface 12 by conventional corona discharge charging means (not shown) and exposing the photoconductive layer to a light image from a light source (not shown), either indirectly as reflected from a document or directly from a cathode ray tube. However, a photoconductive layer is not required and the electrostatic charge pattern can be selectively deposited by charging means, such as a pin tube, known in the art.

For developing the electrostatic charge pattern carried on the member 11, a development station 15 is provided and employs the novel development means and methods of the present invention. The station 15 comprises a reservoir 16 of developer material 17 which is a mixture of carrier particles and toner powder which electrostatically adheres to the carrier particles. The developer material 17 is carried upwardly from the reservoir 16 to a receptacle 18 via known means such as a feed screw or conveyer buckets, shown schematically by the endless dash lines. A toner dispenser (not shown), which is well known in the art, is included in the development station to continually add new toner to the developer material in the reservoir.

In accordance with the present invention, the development station 15 must include means for creating a finishing development zone from the point of entry of the developer material onto the member 11 to a point upward therefrom whereby development of the electrostatic image is finished or completed in said zone. In the embodiment shown in FIG. 1, the creation and maintenance of the finishing development zone are accomplished in the following way. The receptacle 18 is formed with two rigid walls 19, 20 and a laterally sliding wall 21, which, when moved, changes the width of the throat 22 of the receptacle. In addition, adjacent the receptacle 18 and in the path of the developer fiow onto the member 11 is deflector means 23 for imparting an upward movement to the developer material 17. Thus, as the developer material flows onto the insulating surface 12 of the member 11, a portion of the development material is caused by the flow rate of the developer material, due to the restricted width of the receptacle throat 22, and the deflector means 23 to form a finishing development zone 24 while the remaining portion cascades downward, as shown by the arrow 25, in a counterflow direction to the direction of the member 11.

In this manner, the undeveloped electrostatic charge pattern on the insulating surface 12 of the member 11 first sees the developer material 17 as it enters the development station 15. Thus, the developer material 17 which first comes into contact with the electrostatic image is partially depleted of toner inasmuch as some of the toner was given out to previous electrostatic images. As the electrostatic image continues in the upward direction, further development of the image occurs, but not complete development until it passes through the finishing development zone 24. In this zone, the carrier particles in the lower and middle portions are heavily laden with toner and complete the development of the partially toned electrostatic image. Moreover, the carrier particles in the uppermost position are substantially depleted of toner and, because of this condition, scavage loose toner lying in the background areas of the image. In addition, the path of these substantially toner depleted carrier particles is in a counterclockwise direction back into the path of the incoming developer material where they pick off toner from the incoming toner laden carrier particles and either go back into the zone 24- or are carried downward along the surface 12 into the reservoir 16. To summarize the counterflow development method of the present invention, the electrostatic image is partially developed prior to reaching the finishing development zone where during the initial stages it is completely developed and in the latter stages, the background areas are cleaned of loose toner.

Another embodiment of the present invention is shown in FIG. 2 in which the moving member 11 having an insulating surface 12 takes the form of a drum so that the developing surface is curved rather than planar. Similar to the embodiment of FIG. 1, the development station 15 of FIG. 2 includes a reservoir 16 for providing a supply of developer material 17 to the receptacle 18 via carrying means schematically shown by the endless dash lines. In this embodiment, however, the deflector means 23 of FIG. 1 is not included, but instead the finishing development zone is, at least, partially controlled by means for changing the angle of the chute, herein receptacle wall 19. As shown in FIG. 2, the receptacle 18 is rotatably mounted on a pivot 27 which is slidable in a slotted member 28. The pivot 27 can be moved out of the uppermost slot in the slotted member 28 downward to one of the three other slots. As this is done, the receptacle 18 is rotated upwardly and the wall 19 is moved from its present position to three other positions, each of which places the wall 19 of the reservoir 18 more towards a horizontal attitude. It should be apparent that, as the reservoir wall 19 is moved into a more horizontal attitude, there will be an increase in the size of the finishing development zone. (lonversely, returning the pivot 27 back up to the uppermost slot of the slotted member 28 will cause the receptacle 18 to rotate downwardly and move the receptacle wall 19 back into a lesser horizontal latitude. This will cause a decrease in the size of the finishing development zone 24. While the chute has been shown as the receptacle wall 19 in FIG. 2, it should be understood that the deflector 23 of FIG. 1 can also be similarly mounted so as to change the angle of the deflector 23.

To further illustrate the present invention, a following mathematical contrast between the counter-flow cascade development method and means of the present invention as shown in FIGS. 1 and 2 and the conventional prior art parallel flow cascade development as shown in the prior art figure establishes further advantage of the novel counterflow method over the conventional parallel flow method. The following descriptive equation of cascade development is valid for both counterflow and parallel flow cascade development:

Q=number of carrier particles contacting zone of the member 1.

V =average velocity over the length of the developer zone.

D=length of developer zone.

V =velocity of photoconductor surface.

d=average diameter of carrier particles.

where Substitution in the above equation to practical development parameters, such as a PC speed of 16/sec. and an average carrier velocity of 32/sec. over a developer zone of 4" with carrier particles of .03", a parallel flow method will provide contact with 3216 4 Q=(- 133 particles whereas the novel counterflow method under the same conditions will provide contact with three times more particles, namely While the above mathematical contrast shows the advantage of counterflow cascade development over parallel flow cascade development in the initial development zone, it is required, as previously discussed, that the counterflow cascade development method include a finishing development zone above the point of entry of the developer material onto the insulating member. The velocity of the development material in the finishing development zone is low relative to the velocity of the developer material being introduced onto the insulating surface and gives the visual appearance of being essentially stationary. Accordingly, developer material at the point of entry and in the finishing development zone is being replenished at a rapid rate and consequently a large supply of toner is available for transfer to the electrostatic image on the insulating surface. However, the rate of replenishment of development material is lower in the finishing development zone farthest removed from the point of entry. This serves to enhance the image quality because developer material in this area becomes slightly depleted of toner and picks up loose background toner from the insulating surface. Preferably, for an insulating member traveling at 32 inches per second and having an initial development zone of about 7 inches, the finishing development zone would be in the range of about one-half inch to about two inches. It has been found that, under these conditions, a finishing development zone of less than about a half an inch does not develop the electrostatic image to optimum density and the background of the developed image will contain some loose toner due to the absence of denuded carrier particles in the upper portion of the zone and their background cleaning action. Moreover, under these conditions, if the finishing development zone exceeds about two inches, the developer material in the uppermost portion of the finishing development zone is sufiiciently depleted of toner that it tends to not only clean up the loose toner from the background, but also partially erase the developed image.

Because of the importance of the size of the finishing development zone, it has been found that a number of parameters can be varied to change the size of the finishing development zone. A number of these are as follows: (1) The angle of the chute on which the developer material flows onto the insulating surface (This was previously discussed in the description of FIG. 2.); (2) The size of the throat of the receptacle containing the developer material; (3) The use of a deflector in the path of the developer material flow and the angle of that defiector (This was previously discussed in the description of FIG. 1.); (4) The use of a gate or other restriction means between the insulating surface and the end of the chute; and (5) The angle of the insulating surface.

To be more specific as to parameters (2) and (4), the size of the finishing development zone can be varied by changing the flow rate of the developer material, both out of the receptacle 18 and down the insulating surface 12 of the member 11. That is, the throat 22 of the receptatcle 18 can be increased in size to increase the flow rate and thereby increase in size the finishing development zone. Conversely, the throat 22 can be reduced in size to decrease the fiow rate and thereby reduce in size the finishing development zone. Moreover, the end 26 of the receptacle wall 19 in FIG. 2 can be moved closer to and further from the insulating surface 12 of the member 11 through the action of the slotted member 28 to decrease and increase, respectively, the flow of developer material between the end 26 and the surface 12 and thereby increase and decrease, respectively, the size of the finishing development zone.

An additional parameter which can be varied in order to change the size of the development zone is the angle of the member 11. As shown in FIG. 1, the angle of the member 11 is about 45. If the angle is increased greater than 45, the gravity force on the developer material will be increased, this will reduce the size of the finishing zone, whereas an angle less than 45 will increase the size of the finishing zone because the gravity force on the material will be decreased. Preferably, for web and belt configuration, the angle should be about 45 As pointed out previously, the carrier particles as they reach the uppermost part of the finishing development zone are partially depleted of toner and, because of this, pick up loose undesirable background toner on the image. The degree of the toner depletion on the carrier particles is also dependent upon the size of the finishing development zone. That is, if the size of the finishing development zone becomes too large, the carrier particles become so depleted of toner that they not only tend to pick off toner from the image area but may due to their denuded state accumulate a charge and, because of the polarity of this charge stick to the photoconductor and be carried out of the finishing development zone. In accordance with another aspect of the present invention, means are positioned adjacent the finishing development zone, for providing a controlling force for regulating the toner dcpleted carrier in the upper portion of the finishing development zone so as to prevent such carrier particles from being carried upwardly and out of this zone.

FIG. 3 is an embodiment of this aspect of the present invention and shows a voltage source applied to an insulated mounted chute in the form of a deflector 23. The potential applied to the chute brings about a change in the net charge of carrier particles when they become denuded or depleted of toner such that they do not stick to the surface of the member 11 which, for illustration purposes, has been shown as a solid curved line in the form of a drum and a straight dashed line in the form of a web or endless belt. The electrical potential applied to this chute must be kept at a polarity opposite to the triboelectric charge on the carrier particles and at a voltage level sufficient to prevent the toner depleted carrier particles from being carried upwardly and out of the finishing development zone. Normally, this is about 200 volts and, for example, if the electrostatic pattern is negative, the triboelectric charge on the carrier particles will be negative and the polarity of the potential applied to the chute will be positive. It will be apparent, by this technique, that the core of the carrier particles may be either insulating, such as glass or sand, or conductive, such as iron.

Another embodiment for regulating the toner depleted carrier in the uppermost portion of the finishing development zone is shown in FIG. 4. Herein, a non-magnetic cylinder 29 rotates about a stationary magnetic hub 30 having one or more axially aligned strip magnets 31 adjacent the inside of the rotating cylinder. The cylinder 29 is rotated in a direction opposite to the movement of the member 11 and in the direction of the arrow 32. Magnetic fields originating from the strip magnets 31 of the hub extend through the nonmagnetic cylinder and attract the toner depleted carrier particles having magnetically attractable cores, such as iron. The rotational direction of the cylinder 29 carries the carrier particles back into the developer flow from the reservoir 18 and out of the magnetic fields whereby the toner depleted carrier particles are released into the flow and become retoned from the toner laden carrier particles fresh from the reservoir. Accordingly, not only does the rotating cylinder through the magnetic fields carry denuded or toner depleted car rier particles back into the developer flow, but it also can be used to control the number of toner depleted carrier particles in the upper portion of the finishing development zone. That is, by increasing the rotational speed of the cylinder, the number of such carrier particles will be decreased in the zone due to the increased number of them being carried out of the zone and back into the flow of developer material. It should be apparent that further control over the size of the zone can be accomplished through the field strength of the magnetic field.

In addition, if the cylinder 29 is metallic, a potential can be applied to it from a voltage source 33 and perform a further function as a development electrode for improving solid area development of the electrostatic image. The potential applied to the cylinder must be slightly greater than the background voltage level and substantially less than the voltage level of the image or pattern. For example, negative electrostatic charge pattern normally has a surface potential of --700 volts in the pattern areas and a potential of -l volts in the background areas. Thus, the voltage applied to the cylinder 29 should be about l50.

From the foregoing, it can be seen that, because the insulating member carrying the electrostatic image is moving in a direction opposite to the direction of the developer flow, the initial velocity of the developer material can be very low relative to parallel flow development and the initial contact of the developer material with the surface of the insulating member is more of sliding action and, therefore, non-destructive to the surface. Moreover, the insulating member receives a polishing action during the development method of the present invention, which is believed to be due to the sliding action of the developer material against the insulating surface. In addition, it should be noted that, in contrast to parallel fiow development, the developed electrostatic image leaves the upper end of the developer station, it developed image next proceeds to a transfer station in which the image is transferred to a substrate such as paper, the transfer station will be located at the top of the electrostatographic machine. Because of this, the design of the paper handling is simplified due to the short path length for incoming plain paper to the outgoing copy paper carrying the develoed image. Moreover, it is on a convenient level for the operator of the machine.

While the invention has been shown and described with reference to preferred embodiments thereof, it will be appreciated by those skilled in the art that variations in form may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electrostatographic method in which an electrostatic image is formed on an insulating surface, the developing of said electrostatic image, while said insulating surface is moving, to render it visible, comprising the steps of:

moving said insulating surface carrying the electrostatic image in an upward direction;

providing a flow of developer material comprising carrier particles and toner powder onto the moving insulating surface; and

flowing said developer material downward and in a direction substantially opposite to the direction of movement of said insulating surface so that develop ment of the electrostatic image begins at a distant point downward from the point of entry and at a rate such that a finishing development zone of developer material is formed upward from the entry of the developer material and of a size sufiicient to finish development of partially developed portions of the image and to pick off loose toner powder from the background.

2.- The method of claim 1 wherein said fiow of the developer material onto said moving insulating surface is deflected upwardly to assist in the formation of said finishing development zone.

3. The method of claim 1 wherein said flow of the developer material onto said moving insulating surface is capable of regulation so as to permit the size of said finishing development zone to be varied.

The method of claim 3 wherein the size of said finishing development zone is regulated such that the developer material in the upper portion of the zone includes partially toner depleted carrier particles which pick off loose toner powder from the background of the developed image.

5. The method of claim 4 wherein a controlling force is applied adjacent said finishing development zone so as to prevent the toner depleted carrier particles from being carried upwardly and out of the zone.

6. The method of claim 5 wherein said controlling force 18 an electric field opposite in polarity to the triboelectric charge on the toner depleted carrier particles so as to change their net charge and thereby prevent them from electrostatically sticking to the insulating surface and being carried upwardly and out of said zone.

7. The method of claim 5 wherein said controlling force is a magnetic field and said carrier particles comprise a magnetically attractable material whereby, through magnetic attraction of the toner depleted carrier particles to a rotating member, the particles are carried back into said flow of developer material.

References Cited UNITED STATES PATENTS 10 Caldwell et a1 1l717.5 Cerasani et al 11717.5

Anderson 117'17.5 To'ku Hojo et a1. 11717.5

Motoki Kojima et a1. 11717.5

MURRAY KATZ, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R. 

